It is known, for example, to illuminate the keypad of a mobile telephone in an automobile to facilitate night-time use. Each individual key may be illuminated by a respective light source located directly behind the key. This arrangement is, however, very expensive since a separate light source is required for each key. Also, without carefully designed diffusers in or behind the keys, unsightly bright spots may be seen through the keys coinciding with the light source.
A more economical approach utilizes fewer distributed light sources and the light is directed towards the keys with the aid of a light guide in the form of a transparent plate. A known light guide comprises an array of apertures above which a respective translucent key may be located As is conventional, the key may carry a numeric and/or alphabet character, a symbol, a word or an abbreviated message A portion of the key depends or extends into the aperture so that light can be coupled into the key from the light guide. A first conical indentation feature is present adjacent one surface of the plate at the interstices of a group of four apertures. A light source may be accommodated at least partially in the indentation which is effective to direct light into the plate generally in the direction transverse to the plane of the plate. A second conical indentation is present at the opposite surface of the plate and aligned with the first cone in such manner that the apexes of the two cones confront each other. The second cone is effective to disperse light from the first cone by total internal reflection omnidirectionally into the plane of the plate. Thus some of the light is directed towards the apertures and so illuminates the keys. A drawback with this arrangement is that it tends not to give uniform illumination and the lighting effect deteriorates particularly towards the edges of the keyboard or keypad and generally at locations further away from the light sources.
U.S. Pat. No. 3,774,021 discloses a light guide for illuminating a telephone dial faceplate having twelve apertures using two distributed light sources. In this case a special kind of light source is used which emits light predominantly in a horizontal plane. Therefore these light sources are recessed within the light guide so that the light is coupled directly into the plane of the light guide. The side edges of the light guide are provided with notched re-entrant portions so that light rays incident thereupon tend to be reflected back into the light guide. Hence, the light is both introduced and reflected omnidirectionally within the light guide and it is inevitable that the resulting illumination must lose intensity at points further away from the light sources.
UK Patent GB 2,149,353 discloses an illuminated keypad which does not use a light guide, but instead employs protruding reflectors on the underside of an opaque grid-like key retaining member, which reflect light from distributed light sources directly into the keys.
Hence one light source disposed at the interstice of four keys is responsible for introducing light into all four of those keys. However, it has to be noted that the configuration is such that light is injected primarily at the corners of the keys, and secondly along some, but not all, sides of the keys. Although this configuration may go some way to improving uniformity of illumination with distributed light sources, the lighting effect must nevertheless deteriorate to some extent towards the edges of the keyboard and generally at locations further away from the light sources.
Although it relates primarily to the illumination of instrument panels, e.g. in automobiles, rather than to keyboards, EP-A-0,029,635 discloses a light guide comprising an integral lens feature which is effective to collimate light from a distributed light source before it is reflected into the plane of the plate for illuminating various translucent areas of an information bearing overlay. In this case the light is reflected omnidirectionally (by internal reflection into the plane of the light guide) by a 45.degree. semi-vertical angle conical recess aligned with the lens but on the opposite face thereof.